DNS trouble ISPC v2 and 3

i got some trouble whit DNS.
The main ISPC i v2 and it got mx=mail.mydomain.se and all working fine.

Then i install ISPC v3 whit mx=s10.mydomain.se and it working nice, i add a new domain newdomain.se and in DNS record i added s10 as mx server, all working fine until the newdomain.se mail mydomain.se then i got the following error in the mail
(the 2 ISPC v2 and v3 is using different routers but are on the same subnet)

This is the mail system at host s10.mydomain.se.

I'm sorry to have to inform you that your message could not
be delivered to one or more recipients. It's attached below.

For further assistance, please send mail to postmaster.

If you do so, please include this problem report. You can
delete your own text from the attached returned message.